The easiest way to learn how to use latex is to look at how other people use it. Here is a list of real world latex sources that are freely available on the internet. The information here is sorted by application area, so that it is grouped by the scientific communities that use similar notation and LaTeX constructs.

Articles on programming language research, from syntax to semantics, including source code listings, type rules, proof trees, and even some category theory. A good place to start is Mitchell Wand's Latex Resources, including a sample file that also demonstrates Didier Remy's mathpartir package. The following are latex sources of some articles, books, or presentations from this field: